In recent years, with the general improvement in motor vehicle performance accompanying increases in engine output among other factors, improvement has been sought in acceleration performance, particularly in the acceleration performance from standstill (hereafter referred to as starting performance), leading to the demand for a torque converter having a high (maximum) stall torque ratio.
In general practice, when designing a torque converter, the capacity factor is first determined considering its compatibility with the engine to be used, then the radial dimension of the torque converter, and the shapes of the impeller and turbine blades are determined on basis of the capacity factor. Meanwhile, the dimension of the torque converter, particularly the axial dimension, is determined from the restrictions imposed on its mounting on the vehicle. Hence, the flatness ratio L/H is determined from the radial dimension and the capacity factor.
Therefore in general, the profile of the stator blades has to be altered, increasing the difference between the inflow and outflow angles, in order to obtain a higher stall torque ratio.
As shown in FIG. 3, in a torque converter, a speed ratio at which the outflow from the turbine blade 6a is in the direction of no impact with the stator blade 9c is known as the normal speed ratio (e.sub.o). At a speed ratio below the normal speed ratio, the outflow G from a turbine blade 6a impinges the concave face of a stator blade 9c, effecting the required torque conversion (torque conversion range), whereas at a higher speed ratio when the outflow M from the turbine blade 6a matches the outflow angle of the stator blade 9c, no torque is applied to the stator since no change occurs in the direction of flow (coupling point), and at an even higher speed ratio when the outflow H from the turbine blade 6a exceeds the direction of flow M at coupling point, the outflow impinges on the convex face of the stator blade 9c, causing the stator to rotate freely by overriding the one-way clutch 7 (coupling range).
If the camber of the stator blade 9c is increased from that indicated by broken lines to that indicated by solid lines in order to obtain a higher stall torque ratio, the outflow from the turbine blade 6a in the direction of no impact with the stator blade 9c, corresponding to the normal speed ratio (e.sub.o), would shift from 0.sub.1 to 0.sub.2. This results in the shifting of the torque conversion range to a lower speed range, resulting in reduction in efficiency .eta., particularly in the maximum efficiency .eta..sub.max. Hence in the torque converters of prior art described above, the stall torque ratio could not be raised if efficiency is to be maintained, and therefore has remained at relatively low values.
The object of the present invention is to provide a hydraulic torque converter which enables the selection of a high stall torque ratio without loss in efficiency, while maintaining the flatness ratio (L/H) within the range 0.82-0.9 to meet the mounting restrictions.